Articles containing colorants are known to lose their color when exposed to solar radiation for extended times. In particular, fluorescent colorants degrade more quickly than conventional colorants, often turning colorless on exposure to daily solar radiation in a matter of days or months. Even though they are less durable, fluorescent dyes are commonly used for increased visibility of an article due to the visual contrast between a dyed article and its surroundings. Increased visibility is particularly important, for instance, in the traffic sign industry. Fluorescent colored signs have been shown to increase motorist and pedestrian safety, but their use remains limited due to their poor color stability and the need to frequently replace them in order to maintain effective performance.
Attempts to maintain color of fluorescent articles have included adding ultraviolet (UV) overlay screens which effectively filter radiation below 380 nm. Such protective overlays add cost and complexity to otherwise low-maintenance articles. Hindered amine light stabilizers (HALS) have been added to polycarbonate matrixes to enhance the durability of fluorescent dyes contained therein.
Interpenetrating polymer networks (IPNs), systems comprising two independent crosslinked polymer networks, have been described. See, for example, Encyclopedia of Polymer Science and Engineering Vol. 8; John Wiley & Sons, New York (1987) p. 279 and L. H. Sperling, Introduction to Physical Polymer Science, John Wiley & Sons (1986) pp. 46-47. In particular, IPNs comprising acrylate and urethane networks have been prepared by either sequential or simultaneous (but independent) polymerization of free-radically polymerizable ethylenically-unsaturated acrylate-type monomers and urethane precursors (i.e., polyisocyanate and polyhydroxy coreactants). See, for example, U.S. Pat. Nos. 4,128,600, 4,342,793, 4,921,759, 4,950,696, 4,985,340, 5,147,900, 5,256,170, 5,326,621, 5,360,462, and 5,376,428.
Single phase polymers comprising pigments or dyes have been disclosed. See, for example, U.S. Pat. Nos. 3,253,146, 5,605,761, and 5,672,643.